The physiological characteristics of the circular and longitudinal muscles in the uterus of rats between day 18 and parturition will be compared with respect to: 1. The influence of stretch, placental hormones, and changes in extracellular Ca/Na. 2. The sensitivity of the two muscle layers to exogenous catecholamines. 3. The role of extracellular calcium in catecholamine-induced contractions of muscles. Electrical activity will be recorded with microelectrodes and the tension with force transducers.